134 REPORT uniti:d states geological survey. 



It is about 250 Icet lii.i^ii, lias u very uanow base, and forms Ji ])roiaiiieiit 

 feature iu tiie laudscape. Soutlieast of the mountain we observe a num- 

 ber of similar buttes, which ai)]>roximately follow a course of east 45^ 

 south. Most of them are table-topped hills, from GOO to 1)00 feet in height, 

 showing- volcanic caps of varying thicknesses. They maj^ be estimated 

 at 200 to 700 feet. 



More interesting than the table-topped hills Avas one occurrence at 

 liasalt Spiings. There a ridge, trending south of east and composed of 

 Wasatch beds, was covered with an extensile flow of basaltoid rock. 

 On the summit of the ridge, three cone-shaped hills were noticed 

 standing in its strike. One of these presented the aspect of a crater, 

 while the other two resembled tlie highest point of Mount Essex. Along 

 the northern slope of the ridge the volcanics were broken off abruptly, 

 and tumbled down the side in moft chaotic masses. Huge bowlders, 

 weighing hundreds of tons, were piled on top of each other, and com- 

 posed the slope directly below the vertical face of the rock in situ. It 

 was found that a process of undermining has caused a removal of 

 Wasatch beds, and that the overlying lava had fallen down into the exca- 

 vation thus produced. Near the unbroken northern edge of the ridge 

 the last fissure formed by this separation was observed. It extended in 

 a nearly straight line for several hundred yards, Avas over 40 yards deep, 

 and not more than 20 feet Avide at the top, i)ractically closed below. The 

 moA^ement of this huge mass Avas eA'idently arrested by the accumulated 

 boAvlders on the slope a little loAver down, 



Tiiese buttes and hills furnish an indication of the extent of erosion 

 Avhich took place in this region. Through fissures liaA'ing a strike paral- 

 lel among themselves, the laA'a was ejected and poured OA^er the level of 

 the Wasatch, and, in i)art, Laramie strata. Probably the seA^eral areas 

 of eruption Avere disconnected from the beginning, Fluviatile erosion, 

 shaping its course in accordance Avlth the horizontal distribution of this 

 iiard rock, cut deep A^alleys along such ])laces Avhere it could successfully 

 attack the beds. By imdermining the Aolcauics a dimiinition of their 

 area was achicA^ed and they were forced to assume the peculiar forms 

 that at present characterize them. Tendency to columnar structure, or 

 the pronounced deA^elopment thereof, greatly aided the formation of v'er- 

 tical faces in the laA^as. After the masses had fallen, their removal and 

 transportation were effected by the same agents that excaA^ated the A'al- 

 ieys. In the case of the Basalt Springs ridge they were indiAiduaUy 

 too huge to be attacked, therefore retain their original position. 



Upon critical examination we find that the volcanics of these hills 

 show highly interesting features. Some of them are gray and grayish- 

 broAvn, showing a microcrystalline groundmass, with no Aisible segregated 

 mineral but biotite ; others are decidedly basaltic and contain oliAine. 

 During the progress of the Fortieth Parallel SurA^ey this region Avas 

 A'isited and the rocks subjected to microscopical examination. The first- 

 mentioned variety proves to be a leucitophyrite. * It appears that the 

 groundmass is composed entirely of minute crystals of leucite. This 

 occurrence is one of great interest, inasmuch as it forms the first one 

 of this character on the North American continent. In Euroi)e, the 

 leucite rocks have attained great celebrity on account of their unique 

 character as Avell as on account of the beautiful crystals they contain. 



By the discoA^erers of the peculiar mineralogical constitution of these 

 ci'uptiA'es, the entire series of hills has been named "Leucite Hills," At 

 Mount Essex I obserA^ed the most typical leucitophyrite. It seems to be 



* U. S. Geol. Surv. Fortietli Par., vol. vi, p. 260, 



